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Abstract

Background

Carbon storage in deep saline reservoirs has the potential to lower the amount of
CO2 emitted to the atmosphere and to mitigate global warming. Leakage back to the atmosphere
through abandoned wells and along faults would reduce the efficiency of carbon storage,
possibly leading to health and ecological hazards at the ground surface, and possibly
impacting water quality of near-surface dilute aquifers. We use static equilibrium
and reactive transport simulations to test the hypothesis that perturbations in water
chemistry associated with a CO2 gas leak into dilute groundwater are important measures for the potential release
of CO2 to the atmosphere. Simulation parameters are constrained by groundwater chemistry,
flow, and lithology from the High Plains aquifer. The High Plains aquifer is used
to represent a typical sedimentary aquifer overlying a deep CO2 storage reservoir. Specifically, we address the relationships between CO2 flux, groundwater flow, detection time and distance. The CO2 flux ranges from 103 to 2 × 106 t/yr (0.63 to 1250 t/m2/yr) to assess chemical perturbations resulting from relatively small leaks that may
compromise long-term storage, water quality, and surface ecology, and larger leaks
characteristic of short-term well failure.

Results

For the scenarios we studied, our simulations show pH and carbonate chemistry are
good indicators for leakage of stored CO2 into an overlying aquifer because elevated CO2 yields a more acid pH than the ambient groundwater. CO2 leakage into a dilute groundwater creates a slightly acid plume that can be detected
at some distance from the leak source due to groundwater flow and CO2 buoyancy. pH breakthrough curves demonstrate that CO2 leaks can be easily detected for CO2 flux ≥ 104 t/yr within a 15-month time period at a monitoring well screened within a permeable
layer 500 m downstream from the vertical gas trace. At lower flux rates, the CO2 dissolves in the aqueous phase in the lower most permeable unit and does not reach
the monitoring well. Sustained pumping in a developed aquifer mixes the CO2-affected water with the ambient water and enhances pH signal for small leaks (103 t/yr) and reduces pH signal for larger leaks (≥ 104t/yr).

Conclusion

The ability to detect CO2 leakage from a storage reservoir to overlying dilute groundwater is dependent on CO2 solubility, leak flux, CO2 buoyancy, and groundwater flow. Our simulations show that the most likely places to
detect CO2 are at the base of the confining layer near the water table where CO2 gas accumulates and is transported laterally in all directions, and downstream of
the vertical gas trace where groundwater flow is great enough to transport dissolved
CO2 laterally. Our simulations show that CO2 may not rise high enough in the aquifer to be detected because aqueous solubility
and lateral groundwater transport within the lower aquifer unit exceeds gas pressure
build-up and buoyancy needed to drive the CO2 gas upwards.

Graphical abstract

Background

Carbon storage as a liquid, gas, dissolved carbon, or as carbonate minerals has the
potential to significantly offset global warming caused by anthropogenic combustion
of fossil fuels [1,2]. It is generally accepted that the most suitable systems for geologic storage are
depleted oil and deep saline reservoirs, because they are not viable for domestic,
industrial, and agricultural uses and they are separated from the atmosphere by 1000s
of meters of geologic strata. Despite the physical separation between a storage reservoir
and a useable aquifer, there is still concern that storage reservoirs may leak through
abandoned wells or along faults and be released back to the atmosphere. Leakage of
supercritical CO2 at depth will change to its gas state at lower pressures associated with shallow aquifers,
and may be accompanied with deeper formation water. Leakage would reduce the efficacy
of carbon storage, possibly leading to health and ecological hazards at the ground
surface, and possibly negatively impacting water quality of near-surface dilute aquifers
[3-8]. In order to assess risk and long-term liability it is important to utilize sensitive
monitoring techniques that both detect leaks and quantify the magnitude of the leak
[8].

Carbon dioxide leakage rates that may compromise storage effectiveness and safety
can range over several orders of magnitude. We have chosen to use units of metric
tons per year (t/y) to directly compare leakage rates with the amount of CO2 that may be injected from a large coal plant. At the lower end of the scale, a detectable
anthropogenic CO2 flux from the subsurface must be greater than about 102 t/yr to exceed CO2 produced by biological mineralization of organic matter [9-12]. Anthropogenic CO2 fluxes of 103 to 105 t/yr are above background and represent small leaks corresponding to only 0.01% to
1% of the annual amount of CO2 injected to the subsurface for a gigawatt coal plant that generates and sequesters
107 t/yr of CO2. Although these leakage rates are small relative to the amount of CO2 that can be stored in deep reservoirs, they are comparable with CO2 fluxes from abandoned wells to natural CO2 reservoirs and volcanic activity [7,13,14]. Slow leakage of CO2 gas from small leaks can present a hazard if it builds up in enclosed areas like the
basements of buildings or holes in the earth. This would include fault leaks and slow
well leaks, and has been the cause of ecological hazards and of human fatalities in
areas with volcanic emissions of carbon dioxide [15,16]. Higher carbon dioxide release rates on the order of 20% of the annual amount of
injected CO2 are equivalent to the largest reported amount of CO2 released from catastrophic well failure that was mitigated within seven days [17]. Water chemistry associated with CO2 gas leakage into domestic groundwater resources maybe an important way to monitor
the potential for further leakage to the surface over a wide range of fluxes, because
the acidity associated with dissolved CO2 will alter the ambient water chemistry.

In this paper we use static equilibrium and reactive transport simulations to test
the hypothesis that perturbations in water chemistry associated with a CO2 gas leak into dilute groundwater are important measures for the potential release
of CO2 to the atmosphere. Simulation parameters are constrained by groundwater chemistry,
flow, and lithology from the High Plains aquifer. The High Plains aquifer is used
to represent a typical sedimentary aquifer that may overly a deep CO2 storage reservoir. Specifically, we address the relationships between CO2 flux (103 to 2 × 106 t/yr), detection time and detection distance.

Static Equilibrium Model

In this section we establish that increases in pCO2 perturb groundwater chemistry of domestic aquifers by comparing the geochemical signatures
of the High Plains aquifer and a model aquifer column saturated with CO2(g) as a function of depth. This geochemical model is incorporated into the reactive
transport calculations that describe the ability for detection of stored CO2 that has leaked into a domestic aquifer.

Methods

Equilibrium calculations were conducted using the Geochemist's Workbench software [18], the thermodynamic data listed in Table 1, and the Debye-Huckel activity coefficients to correct for ionic strength. The model
aquifer volume is a 1 m2 × 200 m column containing 30% porosity, 66.5 vol % quartz sand, and 3.5 vol % calcite.
Calculations are made from 40 m to 240 m, with a pH 7.6, 0.01 m NaCl background electrolyte
at 17°C to be consistent with the depth interval for the saturated zone, temperature,
and ionic strength reported for the High Plains aquifer [19-21]. Bicarbonate concentration, HCO3- is adjusted to maintain charge balance, and pore waters are in equilibrium with respect
to calcite and quartz. Silicate dissolution and precipitation kinetics and cation-exchange
to layered silicates are not considered because their contribution to groundwater
chemistry is expected to be minimal compared to carbonate chemistry over the short
time periods associated with leak detection (days to months). Calcite dissolution
kinetics were not included in the simulation because preliminary calculations show
that even at equilibrium only a small amount of calcite dissolves, as is discussed
below. In these calculations we assume that the pore space is fully saturated with
water and that the aquifer volume can accommodate increases in pCO2 equal to the total hydrostatic pressure of the aquifer (hydropressure gradient = 0.09667
atm/m below the water table).

Geochemical response of CO2 leak

Figure 1 compares the simulated geochemical response of a model sandstone aquifer saturated
with respect to CO2 gas with the ambient groundwater chemistry measured from the High Plains aquifer from
40 to 240 m below the ground surface. Groundwater chemistry for sites within the southern
and central High Plains aquifer represents a reasonable range of water chemistry for
dilute aquifers that are likely to be some 800 to 2000 meters above deep carbon dioxide
storage sites. The study areas in the southern and central High Plains aquifer are
primarily within the Ogallala Formation consisting of sands, gravel, siltstone and
clay, and calcium carbonate cement [20,21]. Although both study areas have fairly dilute groundwater, the dissolved solids and
alkalinity are about ten times more concentrated in the central study area than in
the southern study area. Despite this marked difference in absolute concentrations,
trends in the High Plains aquifer are independent of depth at each site. Groundwater
pH, alkalinity as HCO3-, pCO2 and total dissolved carbon are constant over the sampling depth (40 to 240 m) with
the exception of one data point in the southern study area.

The carbonate signature of the High Plains aquifer is distinct from a model sandstone
aquifer exposed to CO2 gas that might leak from a much deeper storage reservoir. Figure 1 plots the pH, alkalinity as HCO3-, pCO2, total dissolved carbon concentrations, carbonate speciation, and the amount of calcite
dissolved as a function of depth for the aquifer system equilibrated with pCO2 equal to the hydrostatic pressure. Equilibration of CO2 gas that might leak from a carbon sequestration aquifer would alter the ambient chemistry
indicated here by the High Plains aquifer. As the CO2 gas leaks into the aquifer, it dissolves into the solution and drives the pH more
acid and promotes some calcite dissolution according to the following mass balance
reactions:

CO2(g) = CO2(aq)(1)

CO2(aq) + H2O = HCO3- + H+(2)

CaCO3(calcite) + H+ = Ca2+ + HCO3-(3)

The chemical response of the aquifer to the leak increases with depth because higher
hydrostatic pressures allow for higher pCO2. Thus solution pH decreases from pH 7.6 to pH 5.2, alkalinity as HCO3- increases from 10-2.6 to 10-1.3 molal, total dissolved carbon increases from 10-2.6 to 1 molal, and the pCO2 increases from 10-2.5 to 101.3 atm from the water table to 240 m below the surface in this calculation. It is clear
from the comparison of the measured High Plains aquifer chemistry and the calculated
aquifer water chemistry, that pH is a robust indicator of an influx of CO2 into a dilute aquifer from a much deeper carbon sequestration reservoir. There is
a shift of about 2 pH units between the measured values in the High Plains aquifer
and those in the leak simulation at depths 60 m below the water table. Although calculated
HCO3- values fall within the range measured in the High Plains aquifers, alkalinity is equally
important parameter to measure because when combined with pH it can be used to calculate
the pCO2 (sum equations 1 and 2):

CO2(g) + H2O = HCO3- + H+(4)

to confirm that the acidity is due to an influx of CO2 and to estimate the magnitude of the leak when measured over time and area. Note that
in dilute-aquifers we assume that the dominant component to alkalinity is HCO3- concentration. This may not be the case in groundwater with significant concentrations
of organic acids or sulfides.

The reason the calculated HCO3- concentrations fall within the range of those measured in the High Plains aquifer
is that only minor amounts of calcite dissolve in response to the acidity generated
by the CO2 leak. About 1% of the calcite dissolved in the deeper portion of the aquifer where
pCO2 is higher, pH is more acid and CO2(aq) is the dominant carbonate species (Figure 1e, f). This suggests that calcite will effectively buffer acidity associated with a carbon
dioxide leak in sandstone aquifers with minor amounts of calcite as well as in limestone
and dolomite aquifers with major amounts of carbonate minerals.

Reactive Transport Model

In this section we explore the effects of CO2 flux, monitoring well location, pumping rate, and gravity-driven groundwater flow
on leak detection using 3-D reactive transport simulations. We assume CO2 gas reaches the aquifer through an abandoned well or along a fault and is then transported
in gas and fluid phases through the sedimentary layers. We use pH as the key geochemical
indicator of carbon dioxide transport in the dilute aquifer. Simulation results are
shown as pH breakthrough curves, contour plots at discrete time steps to capture transport
details between the leak and monitoring well, and selected full-scale simulations
shown as movies in additional files.

Method

The numerical simulations were performed using a parallel-version of the Nonisothermal Unsaturated-Saturated Flow and Transport code (NUFT) to handle large 3-D reactive flow and transport calculations [22]. The NUFT code is a highly flexible software package for modeling multiphase, multi-component
heat and mass flow and reactive transport in unsaturated and saturated porous media.
An integrated finite-difference spatial discretization scheme is used to solve mass
and energy balance equations in both flow and reactive transport models. The resulting
nonlinear equations are solved by the Newton-Raphson method. The NUFT code is capable
of running on PCs, workstations, and major parallel processing platforms. Some of
the application areas include: nuclear waste disposal, CO2 sequestration, groundwater remediation, and subsurface hydrocarbon production [23-26].

We simulate the release of varying fluxes of CO2 gas into a saturated 3-D subsurface flow system that is 10 × 10 km2 × 240 m deep represented by grid size varying from about 50,000 to 150,000 cells with
finer grid spacing near the leak source and the monitoring well. Note that refined
meshes near the evolving interface between CO2 plume and ambient groundwater with dynamic gridding techniques (e.g. adaptive mesh
refinement) were not used because it was beyond the scope and computational budget
of this study. A simulation area of 10 × 10 km2 was chosen to remove boundary artifacts observed for smaller areas due to the extensive
lateral transport of CO2 at high fluxes. A schematic of the subsurface model and the permeability structure
based on the High Plains aquifer is shown in Figure 2. We constrain groundwater flow using lithology depth profiles and the horizontal
hydraulic gradient, because the vertical and lateral heterogeneity of the High Plains
aquifer permeability, porosity, and groundwater flow are not known at sufficient detail
for the scale of the simulations. The High Plains aquifer generally flows from west
to east over approximately 457,000 km2 with an average groundwater flow of 0.3 m/d [27,28]. A 0.3% horizontal hydraulic gradient is estimated from the depth of the water table
documented for four lithology profiles over a distance of 150 km within the central
High Plains aquifer [20,21]. The main aquifer is locally confined by impermeable clay layers near the water table
and at the base of the permeable sandstone units as shown in Figure 2. The unsaturated zone lying above the confined aquifer is also included in the model
for the sake of complete representation of a subsurface system; however, it is not
the focus of the study since the low permeable layer prevents the CO2 vertical transport to the ground surface. Specific lithologies reported in a single
borehole from Liberal, Kansas [20,21] were matched to midrange permeability and porosity values from Freeze and Cherry
[29] to create the permeability and porosity depth profiles used in the simulations, because
direct measurements are not available. The horizontal hydraulic gradient combined
with the permeability and porosity profiles yields an average groundwater flow of
0.3 m/d in agreement with regional groundwater flow for the High Plains aquifer. System
permeability ranges from 10-17 to 2.5 × 10-10 m2 and porosity ranges from 30 to 55%. Infiltration is not explicitly accounted for in
the simulations, but it is reflected in the average groundwater flow of 0.3 m/d.

Figure 2.The 3-D geologic model (10 × 10 km2 and 240 m deep) is based on the central High Plains aquifer sand and clay lithology
[20,21]. The main aquifer unit dips gently with a slope equal to 0.3% and is locally confined
by low permeable clay layers at the top and base of the aquifer.

Table 2 lists the hydrologic properties and parameters used in the reactive transport simulations.
The leak source used in the simulations is small (40 × 40 m2) compared to the size of the storage reservoir and represents a focused leak. The
gas leak is placed just above the lower clay layer at about 170 m below ground surface
into saturated groundwater. The leak is then transported in both the gas and fluid
phases. Gas phase CO2 leakage fluxes equal to 103,104, 105, and 2 × 106 t/yr are used to assess chemical perturbations resulting from leakage rates that may
compromise storage effectiveness and safety as outlined in the background section.
When normalized to the source area, leakage rates range from 0.63 to 1250 t/m2/yr. The temperature is held constant at 17°C throughout the domain. The flow and
transport models are coupled with the same geochemical equilibrium model as described
in Table 1. In the simulation equilibrium conditions are assumed for partitioning components
between the gas and fluid phases. The hydrostatic pressure and constant geochemical
conditions are assigned along the lateral boundary. The model domain horizontal hydraulic
gradient is imposed by changing the gravity vector direction. The relationships between
capillary pressure, permeability, and saturation are described by the van Genuchten
formulation with parameters m and α specified as 0.4 and 6.6 × 10-4 Pa-1, respectively. The gas residual saturation is chosen to be 0.05 and the irreducible
water saturation is 0.2. Initially a very small amount of less condensable gas is
maintained in the aquifer domain (e.g. 0.01 gas phase saturation near the top of the
aquifer and less than 0.002 elsewhere). The presence of the less condensable gas has
no effect on the simulation other than to improve numerical performance (enabling
larger time steps in the simulation).

The effect of groundwater pumping in developed aquifers on CO2 transport and detection is simulated by placing wells (equivalent diameter ~0.56 m)
that are screened at about 110 m in the permeable zone above the CO2 leak source or at 100, 200, and 500 m downstream from the leak source. Pumping rates
range from 0 to 1893 L/min to represent both undeveloped and developed aquifer regions.
Typical irrigation rates for the High Plains aquifer vary from 379 to 1893 L/min (100
to 500 g/min) [27,28].

Overview of CO2 transport

The spatial and temporal development of a CO2 plume is the result of several concurrent processes: CO2 solubility, leak flux, CO2 buoyancy, and groundwater flow, where groundwater flow depends on the hydraulic gradient,
permeability, porosity, and gas saturation. Aqueous solubility transfers CO2 from the gas phase to the groundwater and is in equilibrium with the pCO2 and carbonate mineralogy. Gas pressure and its inherent buoyancy relative to water
transport CO2 gas vertically once the gas pressure exceeds the hydrostatic pressure until the gas
reaches impermeable lithologies such as the clay layers near the water table in the
High Plains aquifer. Gas pressure creates a vertical trace from the leak source to
the top of the aquifer, where CO2 gas saturation builds up and is transported laterally along the base of the impermeable
lithology. Groundwater flow moves the CO2-rich groundwater down gradient from the gas trace. Groundwater pumping for domestic,
agricultural, or industrial uses tends to mix the CO2-affected waters with the ambient groundwater (see Developed Aquifers).

Figure 3 shows that after 6 months of CO2(g) leakage into the water saturated aquifer, gas pressure is slightly elevated over
ambient values within the vertical trace and ranges from 0.3 to 0.6 at the base of
the impermeable layer that defined the pre-leak water table for CO2 fluxes above 104 t/yr. The magnitude of the gas phase saturation increases with the CO2 leak flux rate. CO2(g) saturation is a measure of the relative amounts of gas and water within the aquifer
because CO2 dominates the gas composition in our simulations. The higher gas saturation at the
base of the impermeable layer lowers the water table and results in some downward
groundwater flow. At low CO2 flux = 103 t/yr there is minimal impact of the gas phase on groundwater flow.

Figure 4 and Movie 1 (Additional file 1) provide an example of the development of a pH plume in response to a CO2 leak equal to 105 t/yr at the base of an aquifer unit. We use pH as indicator of CO2 transport because pH is directly related to the pCO2 (Eqn. 4). A vertical plume trace from the leak source to the top of the aquifer evolves
within 6 days as the pressure build-up, caused by the CO2 flux along with buoyancy, rapidly exceeds the local hydrostatic pressure. pH decreases
with depth because the pCO2 is higher at depth in agreement with the static equilibrium model (Figure 1). In the first few days, the CO2 vertical transport is dominant over lateral plume spreading. The low permeable clay
layer at the top of the aquifer acts as a barrier to release to the atmosphere (in
the absence of the confining layer CO2 gas would diffuse through the vadose zone and to the atmosphere [30,31]). Within 40 days the effect of groundwater flow is apparent in the high permeability
layers. This is most obvious as a finger-shaped plume in the most permeable layer
(K = 2.5 × 10-10 m2) in the middle of the aquifer where the monitoring well is screened, but it also
occurs within the slightly less permeable zones within the aquifer. Comparison of
Figure 3 and 4 show that even modest groundwater flow (0.3 m/d) effectively transports CO2 downstream along high permeable zones allowing it to be detected by a change in pH
within a few months. The impact of elevated CO2(g) saturation at the top of the aquifer on downward groundwater flow is shown by
the slight downward expansion of the pH plume from the impermeable layer into the
upper most permeable later (k = 3.0 × 10-11 m-2).

Figure 4.Evolution of pH plume from a CO2 flux = 105 t/yr (62.5 t/m2/yr) and 0.3% hydraulic gradient. CO2 source is at 170 m depth in an aquifer bounded by relatively impermeable clay layers.
Plots show plume details between the CO2 leak source and sampling well at discrete time steps. The full lateral extent of the
plume is shown in Movie 1 (see Additional file 1). The plots are also identified on the pH breakthrough curves in Figure 5.

In our simulations, lateral dispersion at the top of the aquifer and within the highly
permeable layers persists over time, because heterogeneity reflecting fingering and
lateral discontinuities common in sedimentary formations was not incorporated into
the geologic model. It is likely that a pH plume would be more complex when CO2 is diverted by less permeable regions due to lateral heterogeneity present in actual
aquifer lithology. We did not include lateral heterogeneity in the geologic model
because it could not be represented with the available data on the scale of our simulations.

Undeveloped aquifers

In this section we explore the possibility of detecting CO2 leakage from a change in pH in undeveloped aquifers as a function of the leakage rate
and the location of the monitoring well. We define undeveloped aquifers as those in
which there is no sustained pumping for domestic, agricultural, or industrial use.
Figure 5 shows pH breakthrough curves for CO2 flux = 103, 104, 105, 2 × 106 t/yr at 0, 100, 200, and 500 m away from the leak source. In all cases the screened
zone of the monitoring well is within the highly permeable zone (note that positioning
the well at different depths would yield different results). The simulation results
show that the hydraulic gradient is sufficient to transport the CO2 plume so that it can be readily detected by measuring pH at some distance from the
leak source within a few months for CO2 fluxes equal to and above 104 t/yr. As would be expected, breakthrough time decreases as the size of the leak increases
and the distance of the well location decreases. Here, breakthrough is defined as
the time needed for the plume to achieve a pH mid-point between the ambient and the
CO2 affected groundwater at the monitoring well. Plume breakthrough corresponds to pH
= 6.55 for a steady-state pH = 5.5. If the monitoring well happens to be directly
above the leak, then breakthrough decreases from about 30 days at CO2 = 104 t/yr to less than 7.5 hours at CO2 flux = 2 × 106 t/yr. If the sample well is 500 m from the source, then breakthrough decreases from
7.7 months at CO2 = 104 t/yr to 5.7 months at CO2 flux = 2 × 106 t/yr. This is not the case for CO2 = 103 t/yr, where the monitoring well does not detect any change in pH over two-year period
even when the sampling well is directly over the leak source.

Figure 5.pH breakthrough curves in undeveloped aquifers as a function of CO2 flux and distance of the monitoring well from the CO2 leak. CO2 = 103, 104, 105, and 2 × 106 t/yr (0.63 to 1250 t/m2/yr).

The transport of CO2 from small leaks within an aquifer is distinct from higher fluxes, because the pressure
build-up and buoyancy that drive CO2 upward are overcome by aqueous solubility and lateral groundwater flow. This is clearly
seen in Figures 3 and 6 where the CO2(g) plume is limited to the source volume and the pH plume covers a much larger volume.
Figure 6 and Movie 2 (see Additional file 2) shows plume geometry in a series of pH contour plots for CO2 = 103 t/yr. Vertical heterogeneity across the main aquifer yields non-uniform gravity-driven
groundwater flow. Our aquifer model has a 40 m section of fairly permeable sandstone
(K = 5.4 × 10-11m2) at the depth of the leak as seen in Figure 2. The sandstone layer is overlain by silt-rich sandstone with reduced permeability
(K = 5 × 10-12 m2). The pH plume is largely contained within hydrologic layer at the base of the main
aquifer unit, because CO2 transport is controlled by groundwater flow and aqueous solubility. The lateral hydrologic
transport suppresses vertical gas transport in this example. Had the monitoring well
been screened at greater depth towards the base of the aquifer unit, then the pH perturbation
would have been large enough to detect.

Figure 6.Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 0.3% hydraulic gradient. CO2 source is at 170 m depth in an aquifer bounded by relatively impermeable clay layers.
Plots show plume details between the CO2 leak source and sampling well at discrete time steps. The full lateral extent of the
plume is shown in Movie 2 (see Additional file 2). The plots are also identified on the pH breakthrough curves in Figure 5.

Groundwater flow plays an important role in CO2 transport, because it dictates what fraction of CO2 will be transported down gradient as aqueous species within permeable zones and what
fraction will be transported up towards the base of top impermeable lithologies for
a fixed CO2 flux. This is particularly the case for small leaks and is illustrated by comparing
simulation results for CO2 flux = 103 t/yr for scenarios with dip slopes equal to 0.3% and 0.1%, corresponding to average
groundwater flow velocities of about 0.3 m/d and 0.1 m/d, respectively (Figures 6 &7, Additional files 2 &3). Even a small slope change leads to a significant flow-pattern change and directly
affects leak detection by change in pH. The pH contour plot shows that the slower
aquifer flow does not "trap" the CO2 as aqueous species within the lower permeable unit, as is the case for faster flow.
Instead, the pressure build-up along with buoyancy moves CO2 vertically until it reaches the most permeable layer, where CO2 plume moves downstream towards the monitoring well. In addition to transport within
the highly permeable layer, gas pressure and buoyancy continue to drive the CO2 to the base of the clay layers at the top of the aquifer. The lower slope and the
slower groundwater flow yield a pH breakthrough of 6.55 after 1.4 years and pH = 5.7
within 2 years of the start of the leak (Figure 8).

Figure 7.Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 0.1% hydraulic gradient. CO2 source is at 170 m depth in an aquifer bounded by relatively impermeable clay layers.
Plots show plume details between the CO2 leak source and sampling well at discrete time steps. The full lateral extent of the
plume is shown in Movie 3 (see Additional file 3). The plots are also identified on the pH breakthrough curves in Figure 8.

Developed aquifers

Sustained pumping on CO2 transport mixes ambient groundwater with CO2-rich groundwater, where the pH depends on the leakage rate and the location of the
well. For small leaks where the lateral hydrologic transport exceeds gas buoyancy,
sustained pumping enhances leak detection because CO2-rich waters that are being transported along the base of the aquifer are drawn up
to the permeable units where the well is located. Figure 9 shows pH breakthrough curves for CO2 flux = 103 t/yr at 0, 100, 200, and 500 m away from the leak source for pumping rates from 0
to 1893 L/min. In all cases the screened zone of the sampling well is within the highly
permeable zone. In the absence of pumping, lateral transport of CO2-rich water within the sandstone unit at the base of the aquifer is dominant over
vertical CO2 transport towards the top of the aquifer. Thus the CO2 gas never reaches the screen depth of the monitoring well even though the well is
located in the most permeable unit within the aquifer. However, sustained pumping
draws the CO2-rich waters up towards the well yielding earlier pH breakthrough with increased pumping
rate than for undeveloped aquifers. Even though sustained pumping brings the plume
to the well, the simulations suggest that detection of small leaks may be difficult
because steady-state pH of the pumped waters are only about one pH unit lower than
ambient groundwater pH.

Figure 9.pH breakthrough curves in response to CO2 = 103 t/yr (0.63 t/m2/yr) in developed aquifers as a function pumping rate and distance of the monitoring
well from the CO2 leak.

The effect of sustained pumping on plume geometry on small leaks is shown in Figure
10 and Movie 4 (see Additional file 4) for a well with a high pumping rate (1893 L/min) that is 200 m from the leak source.
Initially, CO2 is transported down gradient in the permeable units at the base of the aquifer similar
to case for the undeveloped aquifer (Figure 6). After one year the plume is beneath the well. At this point, the plume is drawn
toward the well, where it is then transported within the most permeable sandstone
unit as well as being removed from the aquifer system by the well. Similar to the
example for undeveloped aquifers, detection of a CO2 leak by measuring pH depends on the depth of the screened well. If the well were screened
at a lower level, then the pH signature would be more acid and easier to detect than
in the example shown here.

Figure 10.Evolution of pH plume from a CO2 flux = 103 t/yr (0.63 t/m2/yr) and 0.3% hydraulic gradient in a developed aquifer with sustained pumping (1893
L/min). CO2 source is at 170 m depth in an aquifer bounded by relatively impermeable clay layers.
Plots show plume details between the CO2 leak source and sampling well at discrete time steps. The full lateral extent of the
plume is shown in Movie 4 (see Additional file 4). The plots are also identified on the pH breakthrough curves in Figure 9c.

For leakage rates greater than 104 t/yr, sustained pumping effectively mixes the ambient water with the CO2-rich water and lowers acidity at the well. Figure 11 shows pH breakthrough curves for CO2 flux = 105 t/yr at 0, 100, 200, and 500 m away from the leak source for pumping rates from 0
to 1893 L/min. Pumping rate has minimal effect on pH breakthrough times when sampled
at a fixed distance from the leak source. The larger impact is that steady-state pH
after breakthrough is more neutral with increased pumping due to mixing of a larger
fraction of the ambient groundwater at greater distances from the leak source, as
well as removing CO2 affected water at higher pumping rates. In practical terms, dilution resulting from
groundwater pumping tends to decrease the likelihood of detecting leaks as the distance
and the pumping rates increase. The steady-state pH of the CO2 affected waters in actively pumped aquifers ranges pH 5.5 for wells above the leak
regardless of the pumping rate to about pH 6.6 units for wells 500 m from the leak
source with high pumping rates (1893 L/min).

Figure 11.pH breakthrough curves in response to CO2 = 105 t/yr (62.5 t/m2/yr) in developed aquifers as a function pumping rate and distance of the monitoring
well from the CO2 leak.

Carbon dioxide leaking from a deep storage reservoir is likely to intercept groundwater
resources before breeching the surface and reaching the atmosphere. The ubiquity of
water wells may provide a simple means to test for such leakage before it reaches
the surface. Occasional chemical testing for pH and alkalinity in water wells would
indicate if carbon dioxide were entering the groundwater and if it is in danger of
reaching the surface nearby. Much of the appeal of using pH and carbonate chemistry
to detect CO2 leakage is that the chemical and hydrological processes governing detection are well
understood and that it uses readily available technology. Groundwater pH and carbonate
chemistry are good indicators for leakage of stored carbon into an overlying aquifer
because elevated CO2 yields a more acid pH than the ambient groundwater. pH and alkalinity are good first
level monitoring tools because these parameters capture the carbonate geochemistry
associated with excess carbon dioxide in a dilute aquifer. It is important that pH
and alkalinity be measured in the field, because their values will change as CO2-rich waters degas and carbonate minerals precipitate [32,33]. A successful monitoring program includes both pre- and post-injection sampling.
It is important to assess the baseline water chemistry and mineralogy in the dilute
aquifer of concern, because identification of a CO2 leak and any associated hazard to the aquifer will be made against the baseline measurements.
Knowledge of the groundwater flow, permeability, porosity and lithology depth profiles
can be used to assess if pre-existing wells can be used for monitoring or if new wells
will be needed.

Reactive transport modeling of site geochemistry and hydrology is a highly useful
for the design of effective MMV plans for carbon storage. Our simulations of the chemical
perturbations associated with a CO2 gas leak into dilute groundwater suggest that more than one monitoring well is needed
to detect leaks, because differences between leak flux, CO2 buoyancy, groundwater flow, and aquifer permeability yield asymmetric plumes over
time. As a result, leaks have a higher likelihood of being detected if the monitoring
well is down gradient from the vertical plume trace and samples the most permeable
units. This is seen in Figures 4, 6, 7, 10 and Movies 1–4 (see Additional files 1, 2, 3, 4), where wells within the most permeable units (K = 2.5 × 10-10 m2) and at the base of the aquifer unit for small leaks (CO2 = 103 t/yr, K = 5.4 × 10-11 m2) would miss the leak altogether if the well is up gradient from the leak trace. One
exception would be wells that sample groundwater near the top of the aquifer just
below the confining layer near the water table (Additional file 1). CO2 spreads laterally along the top of the aquifer, because the confining layer prevents
the CO2 from diffusing through the vadose zone to the atmosphere. The extensive transport
of the CO2 at the top of the aquifer allows CO2 to be detected far from the leak source irrespective of the direction of groundwater
flow. After injection, it is important to sample over time and over a large enough
sampling grid to capture pH breakthrough. In our example, the aquifer contains a very
permeable unit that allows the CO2 leak to be detected for CO2 flux = 104 t/yr within a 15-month time period. CO2 detection in less permeable aquifers might be achieved over longer sampling interval.
These data could then be used in reverse or stochastic simulations to quantify the
location, extent, and magnitude of the leak. A large enough sampling grid over time
ensures that the MMV plan can capture the CO2 leak and its growth, thus avoiding false positive/negative results from single point
measurements. Equally as important, time-series chemical contour maps indicate the
aquifer capacity for the CO2 leak and its potential flux to the surface. As an example, the IEA GHG Weyburn carbon
dioxide monitoring and storage project injected about 5000 t/d of carbon dioxide into
dolomite and limestone oil reservoirs in the southeast corner of Saskatchewan in Western
Canada to study carbon dioxide – enhanced oil recovery at this site and to study monitoring,
site selection, risk and other issues for CO2 storage [34]. The Weyburn project sampled groundwater chemistry prior to CO2 injection and 11 times over a four-year period after injection from about 35 to 60
wells per 25 square kilometers to create the chemical contour maps showing spatial
and temporal trends in carbonate chemistry due to the injection of supercritical CO2 in the reservoir [35,36]. Data of this kind for an overlying aquifer can be used to inform storage efficiency
and future monitoring, remediation, and mitigation programs.

Currently, there are no standards for the containment of stored CO2 in the subsurface. The United States Environmental Protection Agency (US EPA) is currently
proposing that owners and operator's demonstrate that geologic storage of CO2 does not endanger US drinking water for a 50 year-time frame prior to closure of the
site [37]. The US EPA recommends groundwater geochemistry as a monitoring tool, however assessment
of leak magnitude leading to endangerment of drinking waters is still an area of active
research [3-8]. One area of concern is the release of toxic metals in acidified water. Our simulations
show that CO2 generates a slightly acid solution near pH 5.5. Sorption experiments suggest that
a wide range of metals could be desorbed from the iron hydroxides common in sedimentary
rocks as the pH is reduced from ambient conditions to pH 5.5 in response to leaking
CO2 [38]. It is expected that metals would re-sorb to iron hydroxide phases as the solution
is neutralized by reaction or mixing with the ambient groundwater, thus limiting the
long-term hazard. The extent of re-sorption will depend on the concentration of other
ions in solution. For example, CO2 leaks may be accompanied by higher salinity water found in the storage reservoir.
Any metals released at depth or within the drinking water aquifer may remain in the
aqueous phase as chloride or organic complexes. In light of the need to protect drinking
water, it would be prudent to collect filtered, acidified water samples for additional
analysis if required to assess contamination from metals dissolved from hydroxides
or other phases present in the sedimentary rocks, as was seen in the CO2 sequestration field demonstration in the Frio Sandstone [39].

The US EPA regulations on CO2 sequestration do not address minimum standards for leakage on global warming or on
ecological or human hazards should anthropogenic CO2 be released to the atmosphere. The aim is to store carbon dioxide in the subsurface
for 100s of years. Although a range of geophysical techniques are used to track supercritical
CO2 plumes at depth during the injection phase, these techniques are not sensitive enough
to confirm effective storage by detecting small changes in the amount stored. It is
generally believed that "above zone" monitoring is the best approach to account for
the containment of CO2 in the subsurface, because very small releases of stored CO2 will yield large signals. This is clearly the case for CO2 leakage into dilute aquifers. Our simulations show that leakage of ≥ 0.1% (104 t/yr) of the annual amount of stored CO2 from a gigawatt coal fired power plant can be readily detected by changes in pH and
carbonate chemistry (Figures 5, 11). Detection of lower fluxes is possible if the monitoring well grid can capture the
pH plume at greater depths or if active wells draw the plume toward the well. Although
it is possible to detect CO2 using gas samplers at the surface at similar rates, the sampling chambers must be
directly over the surface expression of the leak [7]. The ability to detect CO2 leakage at a distance from the leak source is a key advantage over gas sampling at
ground surface.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

SC provided the geochemical analysis and was the primary author, YH conducted the
reactive transport simulations, and RA provided the larger context for the research.

Acknowledgements

This work was prepared by LLNL under Contract DE-AC52-07NA27344. We thank Andy Tompson
and three reviewers for their comments, which significantly improved the quality of
this manuscript.